This invention pertains generally to the formation of images describing a target area, and more particularly to the formation of such images using reflections of radar pulses.
In some instances, it is important for apparatus on board a guided missile or an airplane to identify objects on the ground without human interaction. For instance, a guided missile fired in the general direction of a target may carry object-identifying apparatus to precisely locate a target. It is known that a synthetic aperture radar (SAR) may be used in guided missiles to locate a target. The SAR produces signals that may be processed by an onboard computer to produce an image that may be analyzed by executing a pattern-matching program to identify a target in the image.
The SAR produces signals by transmitting radar pulses at different positions of the guided missile and receiving reflected pulses from objects on the ground. The reflected pulses at the different positions (forming a long synthetic aperture) provide detail about the target area. The onboard computer then processes reflected pulses at many different positions of the guided missile to produce an image with a resolution that approaches the resolution of a photographic image.
A major difficulty of using a SAR is the requirement for a precise measurement of the phases of the received reflected pulses relative to the phase of the transmitted pulses. The circuitry required to perform such phase comparisons has traditionally been large or costly to manufacture, and therefore ill-suited for use in a small, expendable missile.